US 7376176 B2 Abstract The present invention relates to a method and an apparatus for adjusting an average interval of channel estimation dynamically based on Doppler-shift. The method comprises the steps of estimating Doppler-shift by using level cross rate (LCR) according to differently moving speeds of mobile terminals, determining the optimal average interval of channel estimation based on the relationship between the existing Doppler-shift and the optimal average interval of channel estimation, dynamically adjusting the average interval of channel estimation according to the determined the optimal average interval of channel estimation to make the coherent receiver obtain the optimal estimation performance at different moving speeds. The apparatus of the present invention comprises a channel estimation module, a valid/strongest finger selection module, a RAKE demodulation and combining module, a RAKE output decision module, an LCR detection and Doppler-shift estimation module, a Gaussian noise power estimation module, a strongest path signal power estimation module, and an optimal average interval calculation module.
Claims(3) 1. A method for dynamically adjusting an average interval length of channel estimation, comprising the steps of:
receiving baseband sampling signals and performing a correlation operation to estimate an instantaneous channel parameter and calculate a sliding average estimation value;
selecting an effective signal arriving path and a strongest signal arriving path according to amplitude of the calculated sliding average estimation value;
receiving the baseband sampling signals and performing the correlation operation to obtain a demodulation output in a RAKE demodulation and combining module;
receiving the demodulation output from the RAKE demodulation and combining module and performing a hard-decision on the output from a RAKE receiver to obtain a judgment estimation value of transmitted data symbols;
calculating an average level of the signals selected from the selected effective signal arriving path and the strongest signal arriving path to calculate the average level of the path signal and a detect level crossing rate;
calculating noise power included in the strongest signal arriving path based on the demodulation output and the judgment estimation value of the transmitted data symbols;
calculating a strongest path power estimation based on the calculated sliding average channel estimation value;
dynamically adjusting the average interval length of the channel estimation by selecting an optimal average interval based on the detected level crossing rate, the calculated noise power and the calculated strongest path power estimation.
2. The method dynamically adjusting the average interval of a channel estimation according to
step 1: initiating the parameters, and setting an initial value of average interval parameter as P=P
_{0}, number of iteration is n=1;step 2: calculating instantaneous channel estimation ĉ
_{l,s}(n) to which each pilot symbol corresponds and setting P=P_{n-1}, calculating average channel estimation of a sliding window ĉ_{l,s}(n) using the equation expressed as followsand calculating the average level {circumflex over (R)}
_{l}(n) using the following equationstep 3: calculating statistics for average channel estimation ĉ
_{l}(n−i), where 0<i<K−1in K estimation intervals (i.e. T time interval) and the times for up-cross average level {circumflex over (R)}_{l}(n) that ĉ_{l}(n−i) corresponds to, where 0<i<K−1and estimating Doppler-shift {circumflex over (f)}_{d,l}(n) of mobile station according to following equation:
{circumflex over (f)} _{d,l}(n)≈M _{1}(n)/T; step 4: calculating variance of Gaussian White Noise σ
_{n} ^{2 }with the following equationand calculating the estimated value for the signal power σ
_{l} ^{2 }of the lth path with the following equationand calculating the optimal average length P
_{n }of the channel estimator when Doppler-shift is {circumflex over (f)}_{d,l}(n) with the following equationn←n−1, and then returning back to Step 2.
3. An apparatus for dynamically adjusting an average interval length of channel estimation, comprising:
a channel estimation module for receiving baseband sampling signals and performing a correlation operation to estimate an instantaneous channel parameter and calculate a sliding average channel estimation value;
a valid/strongest finger selection module for selecting an effective signal arriving path and a strongest signal arriving path according to amplitude of the calculated sliding average channel estimation value;
a RAKE demodulation and combining module for receiving the baseband sampling signals and performing the correlation operation to obtain a demodulation output;
receiving a RAKE output decision module for receiving the demodulation output from the RAKE demodulation and combining module and performing a hard-decision on the output from a RAKE receiver to obtain a judgment estimation value of transmitted data symbols;
a level crossing rate detection and Doppler-shift estimation module for calculating an average level of the signals from the selected effective signal arriving path and the strongest signal arriving path to calculate the average level of the path signal and a detect level crossing rate;
a Gaussian noise power estimation module for calculating noise power included in the strongest signal arriving path based on the demodulation output and the judgment estimation value of the transmitted data symbols;
a strongest path signal power estimation module for calculating strongest path power estimation based on the calculated sliding average channel estimation value;
an optimal average interval calculation module for dynamically adjusting the average interval length of the channel estimation by selecting the optimal average interval based on the detected level crossing rate, the calculated noise power and the calculated strongest path power estimation.
Description This Application is a Section 371 National Stage Application of International Application No. PCT/CN02/00605, filed Sep. 3, 2002 and published as WO 03/021989 on Mar. 13, 2003, not in English the content of which is hereby incorporated by reference in its entirety. This invention relates to CDMA (Code Division Multiple Access) cellular communication field, especially to the technique of dynamically adjusting an average interval of Doppler-shift estimation and channel estimation in WCDMA (Wideband CDMA) and CDMA2000-1×. Compared with FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access), CDMA has advantages such as large capacity, anti-multipath fading and high frequency band efficiency, and has become the main technology of radio transmission for 3G (third generation) mobile communications. Spread-spectrum signal receivers of CDMA are divided into two types, i.e., coherent receivers and non-coherent receivers. The coherent receiver needs to know the phase information of received signal, while the non-coherent receiver does not, but it requires that the transmitted signal use orthogonal modulation mode. At present, most of receivers adopt coherent receiving mode. This invention mainly focuses on the coherent receiving mode which is predominant in 3G mobile communication standards. Signal fading exists in the multipath transmission environment of mobile channels. The amplitudes and phases of received signals are time-varying due to the signal fading. In CDMA cellular mobile communication systems, which adopt spread-spectrum technology, the amplitude and phase information of multipath signals can be estimated through receiving the continuous pilot signals with deterministic information. It is therefore possible to achieve multipath diversity and coherent reception. The coherent spread-spectrum receiver that performs diversity process for multipath fading signals is called RAKE receiver. RAKE receivers can perform phase-correction and maximum ratio combining for multipath signals which carry the same information and their fading characteristics are mutually independent, thereby overcoming the multipath fading and improving the received SNR (Signal-to-Noise Ratio). It is necessary to estimate the time-varying parameters of fading channels so as to perform coherent reception. This process is called channel estimation. The conventional method decorrelates each path of received signals by means of the known pilot symbols respectively, estimates a plurality of sampled values of channel parameters and averages the plurality of sampled values to obtain more accurate channel estimation values. In the case where channel parameters keep constant relatively, larger average interval leads to more accurate channel estimation results. In practice, however, channel parameters change with the Doppler-shift of fading channels. The faster the moving speed of mobile terminal is, the larger Doppler-shift is, and the faster the channel parameters change. In other words, the interval in which channel parameters relatively keep constant changes with the moving speed of the mobile terminal (Doppler-shift). The faster the moving speed is, the smaller the interval is, and vice versa. The existing mobile terminals are hard to apply in different mobile environments if fixed average interval is adopted. According to the requirements of 3G mobile communication system, mobile terminals should be able to adapt to the dynamic change in environment such as from static state to moving state of 500 Kilometers per hour. Therefore, in order to obtain optimal receiving performance, mobile terminals should have the abilities of estimating Doppler-shift and adjusting the average interval of channel estimation dynamically according to Doppler-shift. The object of the present invention is to provide a method and an apparatus for adjusting the average interval of channel estimation dynamically based on Doppler-shift estimation, so that the disadvantage, which a fixed average interval is not able to track the speed of a mobile terminal, can be overcome. The method of the present invention dynamically adjusts the optimal average interval of channel estimation based on Doppler-shift. It estimates Doppler-shift by using level crossing rate (LCR) according to variant moving speeds of a mobile terminal, and then, the optimal average interval of the channel estimation can be determined based on the relationship between existing Doppler-shift and the average interval. According to an aspect of this invention, a method of dynamically adjusting an average interval of the channel estimation consists of the following steps: calculating the average level crossing rate (LCR); estimating the channel parameters of the effective arriving paths and the envelope signals thereof by using coherent RAKE reception; calculating the average LCR of the envelope signals of effective arriving path, and estimating the Doppler-shift of channels accordingly; calculating the optimal average interval of channel estimation under current channel environments according to the relationship between the existing Doppler-shift and the optimal average interval of channel estimation; dynamically adjusting the average interval of channel estimation according to the calculated the optimal average interval of channel estimation to make the coherent receiver obtain the optimal estimation performance at different moving speeds. According to another aspect of this invention, an apparatus for dynamically adjusting an average interval of channel estimation comprises a channel estimation module; a valid/strongest finger selection module; a RAKE demodulating and combining module; a RAKE output decision module; an LCR detecting and Doppler estimating module; a Gaussian noise power estimating module; a strongest path signal power estimating module; and an optimal average interval calculating module. Other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the accompanying drawings, in which Firstly, Doppler-shift estimation of CDMA systems in a multipath fading channel environment is described. It is well known from the fading characteristics of wireless channels that the fading rate is relative to the fading depth. The number of times which occur deep-fading is relatively low, and the number of times which occur shallow-fading is quite frequent. Level crossing rate (LCR) quantitatively describes the parameters of this characteristic. Level crossing rate indicates the average number of times which the signal envelop crosses a certain specified level with positive (or negative) slope within a time unit. As shown in In coherent reception of the CDMA system, the continuous pilot channel which is used to transmit the known pilot sequence in advance can be used for system timing, carrier extraction, channel estimation, handover, etc. It is assumed that the total number of effective arriving paths (fingers) which can be distinguished by a RAKE receiver of CDMA system is L, after the received signal passes the RAKE receiver, the output signal of the lth finger after the despreading operation is from equation 2 as follow:
It is necessary to calculate the average level of its channel estimation in order to calculate the Doppler-shift of the ith finger of RAKE receiver. Therefore, the envelop average value of ĉ
Since the transmitted data is performed simultaneously with the code channels, which are parallel with the pilot channel, the channel fading parameters are the same as the those estimated by the pilot channel. After despreading, the lth finger output of the received data channel can be given in equation (8)
The variance σ Step 1: Initiating the parameters, and setting the initial value of average interval parameter as P=P Step 2: Calculating the instantaneous channel estimation ĉ Step 3: Counting statistics for average channel estimation ĉ Step 4: Calculating the variance of Gaussian White Noise σ The iteration method described above is performed on the ith path of the RAKE receiver. Since the effects of Doppler-shift caused by mobile terminals on the arriving path for each channel are the same, The iteration method described above can be simplified to determine the optimal average interval of channel estimation of all L pathes only by using the strongest signal arriving path. The advantages of this invention are given as follows: The method and the apparatus of the present invention can be easily achieved by adding a RAKE output decision module A The invention will be described in detail based on the following embodiments with reference to the attached drawings which do not intend to limit the scope of the present invention. Referring to The method according to this invention can be applied to any CDMA mobile communication systems with continuous pilot channel, including 3GPP (The Third Generation Partnership Project) WCDMA and 3GPP2 (The Third Generation Partnership Project 2) cdma2000 systems. The apparatus according to the present invention can be implemented by adding a RAKE output decision module, an LCR detection and Doppler-shift estimation module, a Gaussian noise power estimation module, a strongest path signal power estimation module, and an optimal average interval calculation module, i.e., five computing units, to a conventional RAKE receiver. As shown in The operation of apparatus according to the present invention will be described in detail as follows. Firstly, the received baseband analog signals are A/D converted to digital signals, and the converted digital signals are input into the channel estimation module B Although the present invention has been described by means of exemplary embodiments, it should be understood that many changes and substitutions may further be made by those skilled in the art without departing from the scope of the present invention which is defined by the appended claims. Patent Citations
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